Lithopone and process of increasing the resistance thereof to sunlight



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This invention relates to the manufacture of lithopone; aiid more particularly it re-,

lat'es to a'process for treatment of lithopone following the calcination step with the object of geatly improving the light resistance'of e pigment.

.In the manufacture of lithopone according to the prior art, the product has been ve li ht sensitive i. e. when ex osed'to the m 8 i P white lead. With the correction of the discoloration, however, another wide field will be opened to lithopone as a. pigment. This comprises its use in outside aints or for other purposes where the fin product will receive more or less exposure to direct sun light.

I'have given azgreat deal of attention to the production 'of a lithopone definitely more light resistant than results from the process known in the prior art. I am familiar with the patents which have appeared from time to time purportin to described a process 'for production 0 light stable lithopones. Most of these cover the addition to the litho- P0118 of some salt capable of precipitating zinc from its soluble compounds.

rocesses I have tried and many of them in act appear at first to be successful. It is true that a great many salts added to the ordinary light sensitive lithopone greatly decrease its graying. In fact almost an reagent from the shelves in the chemica laboratory will have this effect. Unfortunately, however, it is necessary to have a considerable amount of these materials rescut to greatly decrease the graying. t is true likewise that the. pigment will not behave well in paints unless it is washed practically free from water solublesalts. When such washing treatment is ap lied to a lithopone prepared as indicated a ove the grayin tendency returns. In other words, the sa ts referred to protect the lithopone from graying when present in very considerable These Application fled september 5, 1888. Serial Io. 801,088.

mixed with some other aint vehlcle and the ry, it is found that paint film allowed to the salt no longer shows its protective action. Hence, even if it w'erepossible to leave 1n the pigment enough of the added .mater1al to protect from graying in water paste,

the lithoponewould still be useless from the light resistance standpoint when actually apglied in a paint formula.

fter' determining that the above mentioned recesses are in eneral inoperative, I have een able to develop another rocess which actually yields a lithopone muc more light resistant than various lithopones produced according to the prior art. According to my process, crude lithopone is produced in the usual manner by the mutual preci itation of barium sulfate and zinc sulfide om solutions of barium sulfide and zinc sulfate which are prepared in a manner well known in the art. The crude lithopone is filter pressed, dried, mufll'ed, quenched in water and ound wet accordingto the usual practice in the manufacture of ithopone. It is then customary to wash the product with water, add a suitable amount of ultrama'rine or other blue pigment to obtain a satisfactory color, filter press, grind and pack.

According to m invention, however, an additional step is mtroduced which consists in treating the lithopone with an acid reagent. This treatment is preferably made a ter the lithopone has been calcined and wet ground and before it has been washed, although I do not wish to exclude treatment of the unground material or quenching the mufiled pigment in an acid solution, since these treatments would obviously result in the same improvement in the-light resisting properties of the lithopone and therefore possess the same advantage as the preferred method of treatment. I would also include acid treatment at an other step in the process resulting in a li a advantage.

tion, to the lithopone to dissolve a substan-.

tial proportion of the zinc oxide or basic zinc compounds present. By substantial proportionImean enough to give ave detinite test for zinc in the supernatant iquid after the acid treatment. If the original content of the basic zinc compounds is low the amount of zinc dissolved will be rather small. On the other hand if the original content. of basic zinc compounds amounts to say 2% when calculated as zinc oxide, I may dissolve as much as half or more of these com unds.

It will noted that my rocess is distinguished from those cover by the prev1- ously mentioned patents in that the comunds s ecified by the latter are very often scribe as of non-acid reaction. Also their object is often stated to be the precipitation of water soluble zinc compounds initially preselit. It will be seen that m procsee is substantially the opposite of t ese in that I dissolve zlnc compounds instead of precipitating them. Also my reagent is definitely acid rather than non-acid.

As an example of my treatment I may take a wash tub containing 2,000 lbs. of lithopone in the form of a slurry with about 4,000 pounds of water. This lithopone has previously been calcined, quenched in water and wet ground through Buhrstone mills in the manner familiar to those versed in the art. To this wash tub full of lithopone slurry I add a sufiicient amount of. previously diluted sulfuric acid or of aluminum sulfate solution or of a mixture of the two, to give an acid reaction by one of the wellknown indicators. This acid reaction will gradually disappear but it is my custom to add sufli cient of the acid reagent so that the reaction persists after a short period of agitation. It will be understood, however, that I do not limit myself to any special amount of acid reagent inasmuch as a small amount will im rove the light resistance of the product, t is im rovement becoming more and more marks as the amount of reagent added is increased up to a practicable maximum.

The acid treated litho one obtained as described above is then tl ioroughly washed with water so that the content of water soluble salts falls below, say 0.2%. For this purpose 5-10 washings by decantation, for example, is usually sufiicient. When filter pressed, dried and disinte ated in the usual manner my product wil be more light resistant than that obtained by a process which is the same as mine except that the special step of acid treatment is omitted. Such comparison may be made either in water paste or in the form'of a Care must be taken to test both t e treated and untreated pigments under exactly the same conditions. If this is done the improvement due to my process will be obvious.

The washing of the lithopone following the acid treatment need not, of course,,be continued until the water-soluble salt content falls below 0.2%, since a distinct improvement in the litho he will be obtained if the washing be su cient to reduce the water-soluble salt content below 0.5%, or even below 1%.

The acid treatment is generally designed to reduce the acetic acid soluble zinc content, calculated as zinc oxide, to less than 1%, and preferably to less than 0.5%.

As indicated above, it is probable that the solution of the zinc oxide or basic zinc compounds present in the calcined lithopone is responsible for the improvement in its light resistance. However, I wish to make it clear that I am not bound by this theory and that my process for im rovement in the light resistance is success ul regardless of the reason for such improvement and that it is this operative process which I wish to claim. 1

I claim:

In the process of making lithopone the steps which comprise calcining and quenching the crude lithopone, wet-grinding the quenched product, and subjecting the ground lithopone to the action of an acid solution.

2. In the process of making litho one, the steps which comprise calcining an quench ing the crude lithopone, grinding the quenched product, and subjecting the ground lithiopone to the action of dilute sulphuric ac] 3. In the process of making lithopone the step which comprises treating the calcined, quenched, and ground lithopone with suflicient acid solution to dissolve a substantial part of the zinc oxide or basic zinc compounds present.

4. A process as set forth in claim 3 in which the acid solution is a dilute solution of an acid at least as strong as acetic acid.

5. A process as set forth in claim 3 in which the pigment, after being treated with the acid solution, is thoroughly washed to remove not only said acid solution but also any basic zinc compounds which have been dissolved.

aint film...

6. In the process making lithopone which includes the precipitation of crude lithopone, the calcination thereof, and the ='treatin g the lithoponea treating the lithopone after any one of said. operations with a dilute aqueous solution of a reagent of acid reaction until a substan- .which includes the precipitation of crude lithopone, the calcination thereof, and the quenching, grindin and washing of the calcined product, t e step which comprises tial'part of the zinc oxide or basic zinc compounds present has been dissolved, and then removing the resulting solution from the pigment.

8. In the process of making lithopone of improved quality, the step which comprises treating the lithopone subsequent to the calcination thereof with a sufficient amount of an aqueous solution of a rea ent of acid resteps which comprise treating the lithopone.

subsequent to the calcina'tion thereof, with an amount of a solution of a reagent of acid reaction sufiicient to render water-soluble at least a part of the basic zinc compounds present and to reduce the content of the latter, calculated as zinc oxide, to less than 1%, and then washing the thus treated lithopone until the content of water-soluble salts has been reduced to less than 1%. a

10. A process as set forth in claim 9 in which the content of basic zinc compounds is reduced to lessthan 0.5%

11. Aprocess asset forth in claim 9 in which the content of water soluble salts has been reduced to less than 0.5%. '12. A process as 'set forth in claim 9 in which the proportion of basis zinc compounds has been reduced to below 0.5% and the proportion of water-soluble salts has been reduced to less than 0.2%.

13. In the process of making lithopone, the steps which comprise treating the lithopone,.subsequent to the calcination thereof, with an amount of a dilute solution of. an acid sufiicient to renderwater-soluble at least a part of the basic zinc compounds present and to reduce the content of the latter, calculated as zinc oxide, to less than 1%, and' then washing the thus treated lithopone until the content of water-soluble salts has been reduced to less than 1%.

14. A process as set forth in claim 13 in which the content of water-soluble salts has been" reduced to less than 0.5%.

15. A process as set forth in claim 9 in A which the lithopone is treated with a solution containing a salt having an acid reaction.

16. A process as set forth in class 9 in which the lithopone is treated with a solution containing a normal salt having an'acid reaction.

17. The process of making lithopone of improved quality which comprises calcining and quenching crude lithopone, grinding the quenched product, subjecting the ground lithopone to .the action of a dilute solution of a stron mineral acid, and removing the water-solu 1e salts resulting from-the acidtreatment.

18. The process of making lithopone of improved quality which comprises treating the lithopone, subsequent to the calcination thereof, with a dilute aqueous solution of a stron mineral acid, and removin watersolub e salts resulting from said acld-treatment.

In testimony whereof I aflix my si ature.

HERMAN G. SCHAN HE. 

